1. Field of the Invention
The present invention relates to a higher harmonic suppressor element for suppressing a higher harmonic and a DC power supply using the same.
2. Description of the Related Art
FIG. 4 shows a circuit diagram of conventional DC power supply unit. The following will describe the prior art with reference to FIG. 4.
The conventional DC power supply unit 50 is comprised of an AC power supply 52, a voltage selection switch 54 for selecting either one of a 100-V AC voltage and a 200-V AC voltage applied by the AC power supply 52, a rectifying/smoothing circuit 56 for rectifying and smoothing a 100-V AC voltage by the double voltage rectification method if it is selected by the voltage selection switch 54 and rectifying and smoothing a 200-V AC voltage by the equi voltage rectification method if it is selected by the voltage selection switch 54, and a higher harmonic coil 58 which is interposed between the AC power supply 52 and the rectifying/smoothing circuit 56 only when a 200-V AC voltage is selected by the voltage selection switch 54.
The DC power supply unit 50 further includes a filter circuit 60 for canceling noise from an AC voltage, wherein the filter circuit 60 is interposed between the higher harmonic coil 58 and the AC power supply 52. The filter circuit 60 is of a typical type including a coil, a capacitor and a resistor. On an output side of the rectifying/smoothing circuit 56 is provided with a supply voltage control circuit 62 for controlling an output voltage at a constant value. The supply voltage control circuit 62 is a typical type including a switching transistor, an output transformer, a capacitor, a resistor.
The rectifying/smoothing circuit 56 is comprised of bridge-connected rectifying diodes 561-564 and a pair of serial-connected smoothing capacitors 565 and 566. The voltage selection switch 54 is connected between an interconnection point of the capacitors 565 and 566 and one output terminal of the filter circuit 60. The higher harmonic coil 58 has such a frequency characteristic that suppresses a higher harmonic.
The commercially available AC power supply 52 comes in a 100-V type or a 200-V type. To use the 200-V AC power supply 52, the voltage selection switch 54 is opened. In this case, a positive half-wave of the AC voltage flows through the higher harmonic coil 58, the diode 563, the capacitor 565, the capacitor 566, and the diode 562 in this order, while a negative half-wave thereof flows through the diode 564, the capacitor 565, the capacitor 566, the diode 561, and the higher harmonic coil 58 in this order. That is, the 200-V AC voltage is rectified and smoothed by the equi voltage rectification method.
To use the 100-V AC power supply 52, on the other hand, the voltage selection switch 54 is closed. In this case, the positive half-wave flows through the voltage selection switch 54, capacitor 566, the diode 562 in this order, while the negative half-wave flows through the diode 564, the capacitor 565, and the voltage selection switch 54 in this order. That is, a 100-V AC voltage is rectified and smoothed by the double voltage rectification method. Thus, the DC power supply unit 50 gives the same output voltage irrespective of which one of the 100-V and 200-V AC power supplies 52 is used.
Suppose that the voltage selection switch 54 is opened mistakenly when the 100-V AC power supply 52 is used. Then, the rectifying/smoothing circuit 56 rectifies and smoothes a 100-V AC voltage according to the equi voltage rectification method not according to the double voltage rectification method. This causes the supply voltage control circuit 62 to keep the switching transistor ON in an attempt to maintain a predetermined output voltage, thus flowing a mass current to the rectifying/smoothing circuit 56. As a result, an over-current flows also to the higher harmonic coil 58, which is overheated or smokes.
Even when the voltage selection switch 54 is properly set, on the other hand, a short-circuit current may occur owing to a fault or abnormality in the rectifying/smoothing circuit 56 or the supply voltage control circuit 62. In this case also, an over-current flows to the higher harmonic coil 58, which is overheated or smokes.
In view of the above, it is an object of the present invention to provide a higher harmonic suppressor element that can prevent an over-current from flowing to a higher harmonic coil and a DC power supply using the same.
The higher harmonic suppressor element according to the present invention comprising: a higher harmonic coil for suppressing the higher harmonic; a thermal fuse which is connected in series with said higher harmonic coil and also which is melted down by heat generated by said higher harmonic coil. And said higher harmonic coil is contained in an envelope body along with said thermal fuse. Further more, said higher harmonic coil is equipped with a magnetic core and a winding wound around said magnetic core in such a configuration that said thermal fuse is in contact with said magnetic core.
If an over-current flows to the higher harmonic coil, it generates heat to thereby melt down a thermal fuse. With the melting down of the thermal fuse, which is connected in series with the higher harmonic coil, the over-current flow to the higher harmonic coil is cut.
Moreover, a DC power supply unit according to the present invention comprising: a rectifying/smoothing circuit for rectifying and smoothing an AC voltage applied from an AC power supply and the higher harmonic suppressor element which is interposed between said AC power supply and said rectifying/smoothing circuit, wherein the higher harmonic suppressor element including a higher harmonic coil for suppressing a higher harmonic and a thermal fuse which is connected in series with said higher harmonic coil and also which is melted down by heat generated by said higher harmonic coil.
An over-current may flow to the higher harmonic coil owing to a fault or abnormality on the downstream side of the rectifying/smoothing circuit. If an over-current flows to the higher harmonic coil, it runs hot to thereby melt down the thermal fuse. With the melting down of the thermal fuse, which is connected in series with the higher harmonic coil, the over-current flow to the higher harmonic coil is cut.
Further more, a DC power supply unit according to the present invention comprising: a voltage selection switch for selecting either one of a first AC voltage and a second AC voltage which is twice as high as said first AC voltage, wherein each voltage applied from an AC power supply; a rectifying/smoothing circuit for rectifying and smoothing said first AC voltage by the double voltage rectification method if said first AC voltage is selected by said voltage selection switch and, if said second AC voltage is selected by said voltage selection switch, rectifying and smoothing said second AC voltage by the equi voltage rectification method; and
the higher harmonic suppressor element which is interposed between said AC power supply and said rectifying/smoothing circuit only when said second AC voltage is selected by said voltage selection switch, wherein the higher harmonic suppressor element including a higher harmonic coil for suppressing a higher harmonic and a thermal fuse which is connected in series with said higher harmonic coil and also which is melted down by heat generated by said higher harmonic coil.
An over-current may flow to the higher harmonic coil owing to a mistake in setting of the voltage selection switch or an abnormality on the downstream side of the rectifying/smoothing circuit. If an over-current flows to the higher harmonic coil, it runs hot to melt down the thermal fuse. With the melting down of the thermal fuse, which is connected in series with the higher harmonic coil, the over-current flow to the higher harmonic coil is cut.